Various types of equipment including power electronics use solenoids to convert electrical power into mechanical motion. Generally, alternating current (AC) solenoids are used because of the wide availability of AC power from wall outlets. The primary problem with AC powered solenoid is the buzzing encountered when the linear or rotary element is powered into its energized position. The buzzing noise is caused by the cyclic nature of the AC current starting at zero rising to a maximum positive value and then falling through zero reaching a maximum negative value. For example, if common 60 cycles AC current is used, the buzzing would be at a rate of 120 cycles per second. Common problems with this type of AC induced noise occurs in household and office equipment and in specialized applications such as high power electrical contactors used in military equipment.
Direct current (DC) solenoids do not exhibit the aforementioned buzzing noise when their motion producing elements are energized into their extended position. It is known to those skilled in the art that an AC energized solenoid has more pulling force at a given power level than a DC powered solenoid. Thus, if DC power is to be used, the solenoid must be considerably larger or higher, DC currents must be used to generate the same pulling force which can result in excessive operating temperatures. Conversely, it is known that for a given size, a DC solenoid has a higher holding force at full travel than an AC solenoid along with a much lower noise level due to the non-reversing nature of the DC source. Solenoids, such as those disclosed in U.S. Pat. Nos. 4,197,444, 4,520,332 and 3,671,899, the disclosures of which are hereby expressly incorporated by reference, disclose solenoids which use electrically energized coils to produce magnetic fields which act on a rotary or linear element to provide mechanical motion. U.S. Pat. No. 4,544,987, the disclosure of which is hereby expressly incorporated by reference, describes a magnetic switching device which uses an AC power source for energizing for activation of a switching device and subsequentially a DC current for holding the switching device in an activated position. None of these devices use simultaneous continuous energization of both the AC and DC current sources for operation of the solenoid.